Conventionally, electric power supply devices are provided that include a semiconductor switch disposed in a power supply path leading from an electric power source to a load, and turn the semiconductor switch on/off to switch the conduction state of the power supply path. Such power supply devices encompass a power supply device that is provided with an overcurrent interruption mechanism. “Overcurrent interruption mechanism” refers to a mechanism that senses an overcurrent when it flows through the power supply path, and automatically turns the semiconductor switch off to interrupt the current flowing through the power supply path, thereby protecting the semiconductor switch itself and the load connected thereto.
JP 2007-134780A discloses a load driving device that drives an electric component installed in a vehicle. The load driving device includes, in a power supply path through which electric power is supplied from an electric power source to a load, a MOSFET that is arranged in series to the power supply path, and serves as a switch for switching the conduction state of the power supply path. A drain-source voltage of the MOSFET is compared with a predetermined threshold voltage by a comparator. An output terminal of the comparator and the MOSFET are connected to a control circuit, and, upon detecting that the magnitude relationship of the drain-source voltage to the threshold voltage is inverted from that of the normal time, the control circuit determines that an overcurrent has occurred, and automatically turns the MOSFET off. Furthermore, the load driving device of JP 2007-134780A uses several types of resistors having different resistance values to switch the threshold voltage stepwise, and determine several times whether or not an overcurrent has actually occurred, in order to prevent erroneously determining a temporary increase in the amount of current that occurs in normal operation such as an incoming current occurring when the load starts to operate, as an overcurrent.
Furthermore, the load driving device of JP 2007-134780A has a function to diagnose a fault of the overcurrent interruption mechanism. Such diagnosis is performed, when the power supply path is in conduction, such that the device is switched to a diagnosis mode to prevent interruption of power supply to the load, then the resistor that is arranged on an input wire of the comparator is switched so that the threshold voltage increases or the drain-source voltage decreases on a trial basis, and whether the magnitude relationship of these voltages is inverted is checked.
However, in the above-described diagnosis method, even if an operation defect of the overcurrent interruption mechanism is found, it is not possible to identify whether this is a fault of the switch that switches a resistor or a fault of the comparator itself. Therefore, if a defect is found in the above-described diagnosis, there is nothing to be done but perform a standard measure to turn the MOSFET off to interrupt power supply to the load.
Furthermore, in the diagnosis, it is not possible to detect a short-circuit fault of the MOSFET. A short-circuit fault of the switch, which opens and closes the power supply path, is a fault that may lead to a worst case scenario such as a burnout or fire of the load when an overcurrent has occurred, and is also a fault in which fail-safe or fail-soft implementation is difficult because it is not possible to interrupt the power supply path.
In view of the above-described problems, it is an object of the present invention to provide a power supply device that has a function to diagnose circuit operation, and can identify, if a defect is found in the diagnosis, a circuit component that is the cause thereof.